How can I add an ant terrain to an arbitrary shape?
Start with a cylinder with 6 vertices. (a hexagon shape)



Then, add your landscape. Play with the settings until you like how it looks.



Now, add a Boolean modifier on your landscape mesh. Make the operation set to Intersect and the Object set to the 6 sided cylinder we added previously.



Apply the modifier. You may now delete the cylinder. it is no longer needed. Enter edit mode on the landscape mesh, and select the bottom faces using face selection. Using vertex selection, deselect the 6 vertices on the bottom. Then, with proportional editing enabled, scale the vertices that are still selected along the Z axis to 0. Move them down slightly. Delete the vertices at the bottom.



You now have a hexagon shaped landscape.